Current semiconductor technology is based on silicon, gallium arsenide (GaAs), and other opaque materials. But a promising alternative, metal oxide semiconductors (consisting of molecules with both metallic and oxygen atoms) are very efficient, and happen to be transparent. However, fabricating them by ordinary means requires extremely high temperatures, high enough to melt the types of polymers that structure the devices.

A new method for making metal oxide devices at much lower temperatures uses ultraviolet (UV) irradiation. Yong-Hoon Kim and colleagues used UV light to chemically activate metal particles in a chemical solution; the new metal oxide molecules condensed out of the solution, forming a thin semiconducting film. The process can be performed at room temperature—far lower than the 350° temperatures typical of metal oxide fabrication.

Besides the awesomeness factor (itself probably a sufficient reason for most Ars Technica readers), flexible and transparent electronics are potentially useful in medical, transportation, and scientific applications. Conventional semiconductor materials don't work in the thin films required for flexible electronics because they are too brittle. Current organic semiconductors are electronically unstable when stressed and are not efficient at conducting charge, so they are not particularly useful for large-scale implementation.

Metal oxide semiconductors don't suffer from either of these difficulties. They have a high density of electric charge carriers (meaning they are efficient at carrying current) and are amorphous solids rather than rigid crystals. That makes thin metal oxide films very flexible.

Previous experiments have used what is known as a sol-gel technique, where metals (typically indium, gallium, and/or zinc) are dissolved in an organic substance such as 2-methoxyethanol (2-ME). The metal oxide films are formed by annealing: heating the solution (the "sol" part of "sol-gel") to high temperatures, which breaks the molecules of the liquid, allowing its component oxygen atoms to react with the metal atoms. Upon cooling, these metal oxide molecules precipitate out of the solution (the "gel"), forming a thin, transparent film, which can be guided into an etched surface to make circuits.

The high temperatures are the problem. 350°C is above the melting point of most flexible, transparent substances (e.g. plastics), and real electronic devices need a substrate to give them shape. It doesn't matter how thin or transparent metal oxide devices are if they must be deposited on thick, opaque, rigid materials.

The present study bypassed the annealing process by using radiation. UV photons are sufficiently energetic to dissociate some molecules, so the researchers shone intense UV light on the solution. (This sort of light-mediated activation is known as photochemistry.) The solvent was chosen so this freed oxygen atoms, which combined with metal atoms just as in the annealing process, but the film formed at room temperature.

While the deposition of the film happened at room temperature, the mercury UV lamp (a high-intensity version of "black lights") they used heated the film to approximately 150° C, much higher temperatures than expected, which may be a matter of some concern.

The researchers compared the performance of the metal oxides produced by annealing to those made using photochemistry. They found their performance to be comparable, and in some cases the UV-produced semiconductors actually did better in terms of electric charge carrier behavior.

This UV-based photochemistry method of metal oxide semiconductor fabrication is potentially very useful. Even with the unexpected heating, the temperatures were far lower than those required for annealing, which may allow the fabrication of inexpensive, flexible, transparent electronic devices.

Is this technology capable of producing semiconductors of comparable processing power to actual silicon?

In a word, no.

In a paragraph: flexible circuits will never be as dense or power efficient or quick to respond as silicon semiconductors. In modern processors, the speed of light (or more precisely the speed of electrons) governs the physical size of the chip itself since the clock signal must penetrate to all sections of the chip before the next cycle can commence. To make a chip faster it must be smaller, and being smaller it must have more efficient heat removal and so on.

Flexible circuits are primitive. They might give calculators and the Apollo Guidance Computer a run for the money but they're not even on the level of arm and it will be decades if ever before you ever see a flexible film implementation of even the 80386.

With them mentioning MOSFETs as an alternative to current technology (which also uses MOSFETs), the rest of the article is confusing ... they really just seem to mean a special kind of flexible substrate for MOSFETs. Right?

Actually you guys (and the article) are confusing two things -- the type of transistor and the substrate on which it is built.

You can build MOS or CMOS on many different substrates including semiconductors and even insulators (like ruby!). So you can do CMOS on Si, GaAs, SiGe, etc. And also on flexible substrates like the one that isn't properly described in the article.

You can also make other types of transistors on these same substrates, such as bipolar transistors.

You're confusing the gate dielectric (the metal oxide in CMOS) with the active layer.

You can make CMOS with metal oxide substrates and metal oxide dielectrics; the problem is that most of the metal oxides are n-channel conductors because you use dissimilar materials and have some "ionicity," i.e. the valence band and conduction band are centered around oxygen and the metal orbitals, respectively. The metal orbitals overlap well (large s orbitals, which are sphereically symmetric and allow good conducting even in amorphous materials), so you get good conductivity through the conduction band. You can make p-type conducting oxides (copper I oxide, e.g.) but they're not transparent; there are a couple others but they don't perform as well as the n-type TCOs.

However, you don't need CMOS logic when you are actively passing current through a transistor. For example, in a display, the row and column voltages turn on/off another FET. This FET is the driving FET, and when it is on, current flows through them into the LED. Sure, CMOS is great for the multiplexors/etc. that control the row/column voltages, but for the active transistors, it's not necessary.

Also, for displays, you want large areas (you can use DLP or lenses with small LEDs, as other options). Single crystal Si is expensive, so it's not an option for large areas. You can put down amorphous or polycrystalline Si for these active matrix displays, but it's cheaper if you could use metal oxides. Plus, they're transparent for niche applications (flexible, transparent overlays, etc.).

I hope this kind of thing could lead to devices that can be used as a simple wireless medical "bandaid" that you can put on someone's skin. Then, you use an external device record the readings from that "bandaid". Perhaps a series of such devices could provide ECG, EEG and pulse readings wirelessly, to form a crude tricorder type capability.

You're confusing the gate dielectric (the metal oxide in CMOS) with the active layer.

You can make CMOS with metal oxide substrates and metal oxide dielectrics; the problem is that most of the metal oxides are n-channel conductors because you use dissimilar materials and have some "ionicity," i.e. the valence band and conduction band are centered around oxygen and the metal orbitals, respectively. The metal orbitals overlap well (large s orbitals, which are sphereically symmetric and allow good conducting even in amorphous materials), so you get good conductivity through the conduction band. You can make p-type conducting oxides (copper I oxide, e.g.) but they're not transparent; there are a couple others but they don't perform as well as the n-type TCOs.

However, you don't need CMOS logic when you are actively passing current through a transistor. For example, in a display, the row and column voltages turn on/off another FET. This FET is the driving FET, and when it is on, current flows through them into the LED. Sure, CMOS is great for the multiplexors/etc. that control the row/column voltages, but for the active transistors, it's not necessary.

Also, for displays, you want large areas (you can use DLP or lenses with small LEDs, as other options). Single crystal Si is expensive, so it's not an option for large areas. You can put down amorphous or polycrystalline Si for these active matrix displays, but it's cheaper if you could use metal oxides. Plus, they're transparent for niche applications (flexible, transparent overlays, etc.).

That's all very interesting, but you've based your explanation on an incorrect premise that a "metal oxide" is what is being referenced.

CMOS would be better written as Complementary Metal/Oxide/Semiconductor and as such refers to the construction of the transistors with metal gates, oxide gate dielectrics, and semiconductor channels. That was how RCA initially built the process back in the early 1970's, although they called it COSMOS (COmplementary Symmetry Metal Oxide Semiconductor.)

Unfortunately, this no longer actually describes the construction of a modern CMOS process. Depending on the target application of the device, the gate may be metal or poly-silicon, and the gate dielectric may be some form of doped silicon dioxide or not even an oxide at all. The channel is still a semiconductor, though.

I wonder if the elevated temps are due to inefficient heat removal from the UV emitter. If the UV isn't too short a wavelength, maybe a supercontinuum source could be used. The major heat source is at one end of a long fiber optic cable and the rest could be removed with a prism.

Is this technology capable of producing semiconductors of comparable processing power to actual silicon?

In a word, no.

In a paragraph: flexible circuits will never be as dense or power efficient or quick to respond as silicon semiconductors. In modern processors, the speed of light (or more precisely the speed of electrons) governs the physical size of the chip itself since the clock signal must penetrate to all sections of the chip before the next cycle can commence. To make a chip faster it must be smaller, and being smaller it must have more efficient heat removal and so on.

Flexible circuits are primitive. They might give calculators and the Apollo Guidance Computer a run for the money but they're not even on the level of arm and it will be decades if ever before you ever see a flexible film implementation of even the 80386.

I interpret his question differently. Is it capable, in the future, be as powerful as current silicon? I think it will be possible, at least for low power silicon where heat dissipation is not of concern. Speed of electronic devices depend largely on electron mobility, leakage and band-gap characteristics and how densely you can pack them together. The electron mobility for example is why Gallium arsenide is capable of operating much faster than silicon. Nothing says metal oxides can't beat silicon in terms of potential performance. But of course research is required to get there but we don't know enough about these flexible semiconductor to speculate.

Having had a quick look at the paper, the thing that worries me is the high carbon content; the overheat to 150°C will have helped with volatilising the organics, and a significant amount of carbon was remaining after 120mins, barely changing after 30mins.

Off the top of my head I can't think of a better way to remove the organics than heat, so short of increasing T further (say 200-250°C, still much lower than 350°C) or increasing processing time (generally increasing T is more efficient than increasing t, especially when the loss rate tails off so heavily around 30mins), choice of your organic compound is going to be key here - something that does what you want under UV but also has minimal detrimental effects on your metal oxides.

Would it be possible to functionalise them so that they act as electron sinks/wells as applicable in n/p-doped materials?